Pharmacological reduction of pulmonary vascular tone may be a useful modality in the treatment of patients with pulmonary hypertension. While sporadic reports of successful thereapy with vasodilators appear with increasing frequency, skepticism as to their role in routine therapy remains. The possible negative effects of these potential pulmonary vasodilators, along with question concerning their efficacy, have prompted this study which is designed to study patients with pulmonary hypertension as a result of primary pulmonary hypertension (PPH), pulmonary embolic disease (EPH) and chronic obstructive pulmonary disease (COPD). It will test the hypotheses that in these patients pulmonary vasolidators can improve objective measurements of cardiopulmonary function without significant interference with gas exchange, and that this improvement leads to increased functional ability. It is also designed to answer questions concerning the efficacy of different classes of vasodilators, the possible utility of multi-drug thereapy, and the usefulness of various physiological measurement to reflect changes in pulmonary vascular tone. The investigators will utilize three potential vasolidators (nitrates, hydralazine, and calcium-blocking agents) in an unblinded, cross-over study. We will attempt to demonstrate the presence of reversible and reproducible improvements in cardio-pulmonary function secondary to the initiation of treatment. Of particular importance, a sufficient time in the study will be allowed to establish true baselines for both functional performance and cardiopulmonary status. This will hopefully eliminate the ambiquity associated with the results of previous studies.